1. Field of the Invention
This invention relates to tube-diaphragm valves.
2. Description of the Prior Art
Tube-diaphragm valves have long been known and attempts are continuously made to improve their operation.
In one such valve, as for example described in U.S. Pat. No. 4,451,023 (Zakai) a cylindrical valve casing is formed with a pair of end flanges for coupling to inlet and outlet pipes. Located within the casing and directed transverse to the longitudinal axis thereof is an impervious disc like barrier which is supported within the casing by a pervious cage structure which extends on either side of the barrier to respective end portions of the casing. A flexible tubular diaphragm is located coaxially within the casing, is secured at its end portions to respective end portions of the casing so as normally to be stretched over and in contact with the periphery of the barrier, the tubular diaphragm defining with the inner casing surface a control chamber which is adapted to communicate with an outside pressure source via an aperture formed in the casing. In operation, flow takes place through the valve from the inlet to the outlet pipes through the cage structure (formed as a narrow spaced apart array of fingers) and around the impervious barrier as a result of the outward displacement of the tubular diaphragm under the water flow pressure. When this flow is to be interrupted the control chamber is coupled, via the aperture in the casing, to a hydraulic pressure source and, as a consequence, the tubular diaphragm sealingly presses against the barrier periphery cutting off the flow.
With valves of this type the tubular diaphragm is sometimes subjected to excessive control pressure when the valve is in the closed state. This is because when the valve is in the closed state, the downstream pressure in the fluid flow system, in which the valve is coupled, is low as compared with the control pressure in the valve control chamber, and the diaphragm may be stretched excessively in its downstream portion. It is for this reason that the prior art tube-diaphragm valve such as that of Zakai employs a cage-like structure in which the narrowly spaced apart array of fingers is used to support the diaphragm and prevent it from stretching. The array of fingers must be sufficiently strong to withstand the control pressure under the maximum pressure conditions outlined, and yet the array must be constructed with sufficient space between its individual fingers to allow large volume flows sometimes containing solid matter to pass unobstructed. This complicates the design and manufacturing of the cage-like support structure of prior art tube-diaphragm valves.
Repeated forced displacement of the diaphragm towards and away from the central barrier and fingers of the cage-like structure often results in the diaphragm becoming torn and the consequent eventual rendering of the valve unusable.
This vulnerability of the diaphragm to rupture or tearing reduces the pressure ranges within which the valve can be expected to operate effectively.
One way of increasing the operative pressure range of this type of valve is by vulcanizing a cord or a mesh material into the resilient diaphragm material. However, by so doing the resiliency of the diaphragm material is substantially reduced. It has been proposed to overcome this lack of diaphragm resiliency (as for example in U.S. Pat. No. 2,633,154) by using resilient valve housing walls. This proposal has however various inherent drawbacks in connection with the valve housing construction.